The present invention relates generally to reverse osmosis membranes and more particularly to a novel composite polyamide reverse osmosis membrane and to a method of producing the same.
It is known that dissolved substances can be separated from their solvents by the use of various types of selective membranes, examples of such membranes including microfiltration membranes, ultrafiltration membranes and reverse osmosis membranes. One use to which reverse osmosis membranes have previously been put is in the desalination of brackish water or seawater to provide large volumes of relatively non-salty water suitable for industrial, agricultural or home use. What is involved in the desalination of brackish water or seawater using reverse osmosis membranes is literally a filtering out of salts and other dissolved ions or molecules from the salty water by forcing the salty water through a reverse osmosis membrane whereby purified water passes through the membrane while salts and other dissolved ions and molecules do not pass through the membrane. Osmotic pressure works against the reverse osmosis process, and the more concentrated the feed water, the greater the osmotic pressure which must be overcome.
A reverse osmosis membrane, in order to be commercially useful in desalinating brackish water or seawater on a large scale, must possess certain properties. One such property is that the membrane have a high salt rejection coefficient. In fact, for the desalinated water to be suitable for many commercial applications, the reverse osmosis membrane should have a salt rejection capability of at least about 97%. Another important property of a reverse osmosis membrane is that the membrane possess a high flux characteristic, i.e., the ability to pass a relatively large amount of water through the membrane at relatively low pressures. Typically, the flux for the membrane should be greater than 10 gallons/ft2-day (gfd) at a pressure of 800 psi for seawater and should be greater than 15 gfd at a pressure of 220 psi for brackish water. More preferably, the flux for the membrane is at least about 22 gfd at brackish water conditions. For certain applications, a rejection rate that is less than would otherwise be desirable may be acceptable in exchange for higher flux and vice versa.
One common type of reverse osmosis membrane is a composite membrane comprising a porous support and a thin polyamide film formed on the porous support. Typically, the polyamide film is formed by an interfacial polymerization of a polyfunctional amine and a polyfunctional acyl halide.
An example of the aforementioned composite reverse osmosis membrane is disclosed in U.S. Pat. No. 4,277,344, inventor Cadotte, which issued Jul. 7, 1981, and which is incorporated herein by reference. The aforementioned patent describes an aromatic polyamide film which is the interfacial reaction product of an aromatic polyamine having at least two primary amines substituents with an aromatic acyl halide having at least three acyl halide substituents. In the preferred embodiment, a porous polysulfone support is coated with m-phenylenediamine in water. After removal of excess m-phenylenediamine solution from the coated support, the coated support is covered with a solution of trimesoyl chloride dissolved in xe2x80x9cFREONxe2x80x9d TF solvent (trichlorotrifluoroethane). The contact time for the interfacial reaction is 10 seconds, and the reaction is substantially complete in 1 second. The resulting polysulfone/polyamide composite is then air-dried.
Although the Cadotte membrane described above exhibits good flux and good salt rejection, various approaches have been taken to further improve the flux and salt rejection of composite polyamide reverse osmosis membranes. In addition, other approaches have been taken to improve the resistance of said membranes to chemical degradation and the like. Many of these approaches have involved the use of various types of additives to the solutions used in the interfacial polycondensation reaction.
For example, in U.S. Pat. No. 4,872,984, inventor Tomaschke, which issued Oct. 10, 1989, and which is incorporated herein by reference, there is disclosed an aromatic polyamide membrane formed by (a) coating a microporous support with an aqueous solution comprising (i) an essentially monomeric, aromatic, polyamine reactant having at least two amine functional groups and (ii) an amine salt to form a liquid layer on the microporous support, (b) contacting the liquid layer with an organic solvent solution of an essentially monomeric, aromatic, amine-reactive reactant comprising a polyfunctional acyl halide or mixture thereof, wherein the amine-reactive reactant has, on the average, at least about 2.2 acyl halide groups per reactant molecule, and (c) drying the product of step (b) so as to form a water permeable membrane.
The amine salt of Tomaschke is a monofunctional, monomeric (i.e., polymerizable) amine. Preferably, said amine salt is a water soluble salt of a strong acid and a tertiary amine selected from the group consisting of a trialkylamine, such as trimethylamine, triethylamine, tripropylamine; an N-alkylcycloaliphatic amine, such as 1-methylpiperidine; an N,N-dialkylamine, such as N,N-dimethylethylamine and N,N-diethylmethylamine; an N,N-dialkyl ethanolamine, such as N,N-dimethylethanolamine; a bicyclic tertiary amine, such as 3-quinuclidinol; and mixtures thereof, or is a quaternary amine selected from at least one member of the group consisting of a tetraalkylammonium hydroxide, such as tetramethylammonium hydroxide, tetraethylammonium hydroxide and tetrapropylammonium hydroxide; a benzyltrialkylammonium hydroxide, such as benzyltrimethylammonium hydroxide, benzyltriethylammonium hydroxide and benzyltripropylammonium hydroxide; and mixtures thereof.
In U.S. Pat. No. 4,983,291, inventors Chau et al., which issued Jan. 8, 1991, and which is incorporated herein by reference, there is disclosed a membrane which comprises a polymerized reaction product within and/or on a porous support backing material. According to the Chau et al. patent, said membrane may be prepared by contacting a porous support with an aqueous solution of a polyamine which may, if so desired, contain a polar aprotic solvent not reactive with the amines, a polyhydric compound and an acid acceptor. The polyhydric compound, which may include ethylene glycol, propylene glycol, glycerin and other longer carbon atom backbone glycols, may be present in the aqueous solution in an amount ranging from about 0.1 to about 50%. The surface of the coated support is freed of excess solution and thereafter contacted with an organic solution of a polyacyl halide for a period of time sufficient to form a polymerized reaction product within and/or on the support material. The resulting composite is then treated with a hydroxypolycarboxylic acid, polyaminoalkylene polycarboxylic acid, sulfonic acid, amine salts of acids, amino acid, amino acid salt, polymeric acid and inorganic acid, before drying of the membrane.
In U.S. Pat. No. 5,576,057, inventors Hirose et al., which issued Nov. 19, 1996, and which is incorporated herein by reference, there is disclosed a composite reverse osmosis membrane comprising a polyamide type skin layer on a porous support, said membrane being formed by coating a solution A containing a compound having at least two reactive amino groups on the porous support and, thereafter, contacting a solution B containing a polyfunctional acid halide with the coated layer of solution A, wherein the difference between a solubility parameter of solution A and a solubility parameter of solution B is from 7 to 15 (cal/cm3)xc2xd.
Examples of the solvent for solution A set forth in Hirose et al. (""057) are a mixed solvent of water and an alcohol such as ethanol, propanol, butanol, butyl alcohol, 1-pentanol, 2-pentanol, t-amyl alcohol, isoamyl alcohol, isobutyl alcohol, isopropyl alcohol, undecanol, 2-ethylbutanol, 2-ethylhexanol, octanol, cyclohexanol, tetrahydrofurfuryl alcohol, neopentyl glycol, t-butanol, benzyl alcohol, 4-methyl-2-pentanol, 3-methyl-2-butanol, pentyl alcohol, allyl alcohol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propanediol, butanediol, pentanediol, hexanediol, glycerol, etc.; and a mixed solvent of water and a nitrogen compound such as nitromethane, formamide, methylformamide, acetonitrile, dimethylformamide, ethylformamide, etc. As an example of the mixing ratio of water and the other solvent of solution A, Hirose et al. (""057) states that the mixing ratio of water/ethanol can be selected in the range of (50 to 90)/(50 to 10), and preferably (60 to 90)/(40 to 10). Hirose et al. (""057) elsewhere states that the amine salts described in JP-A-2-187135 (corresponding to U.S. Pat. No. 4,872,984), such as a salt of a tetraalkylammonium halide or a trialkylamine and an organic acid can also be suitably used for the solution A to facilitate the film formation, improve the absorption of the amine solution in the support, and accelerate the condensation reaction.
In U.S. Pat. No. 5,614,099, inventors Hirose et al., which issued Mar.25, 1997, and which is incorporated herein by reference, there is disclosed a composite reverse osmosis membrane having a polyamide type skin layer whose average surface roughness is at least 55 nm. The polyamide type skin layer comprises the reaction product of a compound having amino groups and a polyfunctional acid halide compound having acid halide groups. A polymer film may be formed by contacting a solution containing, for example, m-phenylenediamine with a porous polysulfone supporting film so as to form a layer of the solution on the supporting film, then contacting the film with a solution of trimesic acid chloride and holding the film in a hot air dryer so that a polymer film is formed on the supporting film. The surface of the polyamide type skin layer can also be treated with quaternary ammonium salt and coated with a cross-linked layer of an organic polymer having positively-charged groups.
According to Hirose et al. (""099), the compound having multiple amino groups is preferably present in a solution A, said solution A comprising a compound having a solubility parameter of 8-14 (cal/cm3)xc2xd, said compound being selected from the group consisting of certain alcohols, ethers, ketones, esters, halogenated hydrocarbons and sulfur-containing compounds. Specific examples of said compound are disclosed in the Hirose et al. (""099) patent. Elsewhere, Hirose et al. (""099) states that the amine salts described in JP-A-2-187135, such as a salt of a tetraalkylammonium halide or a trialkylamine and an organic acid can also be suitably used for the solution to facilitate the film formation, improve the absorption of the amine solution in the support, and accelerate the condensation reaction.
Although the membranes described above possess a relatively high degree of water permeability, it is nonetheless still desirable to further improve the flux of these types of membranes so that they can be operated at lower pressures, such as at 120 psi, in order to conserve energy while still maintaining a high degree of salt rejection.
Other patents of interest include U.S. Pat. No. 4,950,404, inventor Chau, issued Aug. 21, 1990; U.S. Pat. No. 4,761,234, inventors Uemura et al., which issued Aug. 2, 1988, and U.S. Pat. No. 4,769,148, inventors Fibiger et al., which issued Sep. 6, 1988, all of which are incorporated herein by reference.
It is an object of the present invention to provide a novel polyamide membrane.
According to one aspect of the invention, there is provided a polyamide membrane comprising a reaction product of (i) an aqueous solution comprising a polyfunctional amine, a salt-containing compound and one or more polar solvents, said salt-containing compound comprising at least one amine salt functional group and at least one amine functional group, and (ii) an organic solvent solution comprising an amine-reactive reactant selected from the group consisting of a polyfunctional acyl halide, a polyfunctional sulfonyl halide and a polyfunctional isocyanate.
Preferably, said amine salt functional group is a tertiary amine salt functional group, and said amine functional group is a tertiary amine functional group. Moreover, said salt-containing compound is preferably the reaction product of a strong acid and a polyfunctional tertiary amine. Preferred examples of said strong acid include methanesulfonic acid, toluenesulfonic acid, camphorsulfonic acid, ethanesulfonic acid and benzenesulfonic acid. Preferred examples of said polyfunctional tertiary amine include 1,4-diazabicyclo[2,2,2]octane, 1,8-diazabicyclo[5,4,0]undec-7-ene, N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-1,3-butanediamine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-1,6-hexanediamine, N,N,Nxe2x80x2, Nxe2x80x2, Nxe2x80x3-pentamethyldiethylenetriamine, 1,1,3,3-tetramethylguanidine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine, substituted imidazoles including 1,2-dimethylimidazole and 1-alkyl-substituted imidazoles, and mixtures thereof.
Said one or more polar solvents are preferably selected from the group consisting of ethylene glycol derivatives, propylene glycol derivatives, 1,3-propanediol derivatives, sulfoxide derivatives, sulfone derivatives, nitrile derivatives and urea derivatives. Preferred examples of said polar solvents include alkoxyethanols, such as 2-methoxyethanol, 2-ethoxyethanol, 2-propoxyethanol, and 2-butoxyethanol, 1-pentanol, 1-butanol, di(ethylene glycol) t-butylmethyl ether, di(ethylene glycol)hexyl ether, propylene glycol butyl ether, propylene glycol propyl ether, 1,3-heptanediol, 2-ethyl-1,3-hexanediol, 1,3-hexanediol, 1,3-pentanediol, dimethyl sulfoxide, tetramethylene sulfoxide, butyl sulfoxide, methylphenyl sulfoxide, tetramethylene sulfone, butyl sulfone, acetonitrile, propionitrile and 1,3-dimethyl-2-imidazolidinone.
Said one or more polar solvents are also preferably present in said aqueous solution in a total amount constituting about 0.01 to 8%, by weight, of said aqueous solution.
According to another aspect of the invention, there is provided a polyamide membrane comprising a reaction product of (i) an aqueous solution comprising a polyfunctional amine, a salt-containing compound and one or more polar solvents, said salt-containing compound being a reaction product of a strong acid and a polyfunctional tertiary amine and (ii) an organic solvent solution comprising an amine-reactive reactant selected from the group consisting of a polyfunctional acyl halide, a polyfunctional sulfonyl halide and a polyfunctional isocyanate.
The aforementioned polyfunctional tertiary amine has n tertiary amine functional groups, n being greater than or equal to 2, and said polyfunctional tertiary amine and said strong acid are preferably reacted together in a molar ratio that is greater than or equal to 1:1, respectively, and is less than 1:n, respectively. More preferably, said polyfunctional tertiary amine and said strong acid are reacted together in a molar ratio that is less than 1:(0.95)n, respectively. Even more preferably, said polyfunctional tertiary amine and said strong acid are reacted together in a molar ratio that is less than 1:(0.9)n, respectively.
According to yet another aspect of the invention, there is provided a polyamide membrane comprising a reaction product of (i) an aqueous solution comprising a polyfunctional amine, an amine salt and one or more polar solvents, wherein said one or more polar solvents are selected from the group consisting of 2-ethyl-1,3-hexanediol; a combination of 2-ethyl-1,3-hexanediol and dimethyl sulfoxide; di(ethylene glycol) hexyl ether; a combination of di(ethylene glycol) hexyl ether and dimethyl sulfoxide; di(ethylene glycol) t-butylmethyl ether; a combination of an alkoxyethanol and dimethyl sulfoxide; propylene glycol butyl ether; propylene glycol propyl ether; triethylene glycol dimethyl ether; 1,3-dimethyl-2-imidazolidinone; a combination of 2-ethyl-1,3-hexanediol and acetonitrile; tetramethylene sulfoxide; butyl sulfoxide; methylphenyl sulfoxide; butyl sulfone; and mixtures thereof, and (ii) an organic solvent solution comprising an amine-reactive reactant selected from the group consisting of a polyfunctional acyl halide, a polyfunctional sulfonyl halide and a polyfunctional isocyanate.
Preferably, said amine salt is selected from the group consisting of a quaternary ammonium salt and a reaction product of a strong acid and a tertiary amine. The tertiary amine may be a monofunctional tertiary amine or a polyfunctional tertiary amine.
According to still yet another aspect of the invention, there is provided a polyamide membrane comprising a reaction product of (i) an aqueous solution comprising a polyfunctional amine, an amine salt and one or more polar solvents, wherein said one or more polar solvents are selected from the group consisting of an alkoxyethanol in an amount constituting about 0.05% to about 4%, by weight, of said aqueous solution; dimethyl sulfoxide in an amount constituting about 0.01% to about 8%, by weight, of said aqueous solution; a combination of an alkoxyethanol in an amount constituting about 0.05% to about 4%, by weight, of said aqueous solution and dimethyl sulfoxide in an amount constituting about 0.01% to about 8%, by weight, of said aqueous solution; 1-pentanol in an amount constituting about 0.01% to about 2%, by weight, of said aqueous solution; 1-butanol in an amount constituting about 0.01% to about 3%, by weight, of said aqueous solution; and tetramethylene sulfone in an amount constituting about 0.01% to about 4%, by weight, of said aqueous solution, and (ii) an organic solvent solution comprising an amine-reactive reactant selected from the group consisting of a polyfunctional acyl halide, a polyfunctional sulfonyl halide and a polyfunctional isocyanate.
Preferably, said amine salt is selected from the group consisting of a quaternary ammonium salt and a reaction product of a strong acid and a tertiary amine. The tertiary amine may be a monofunctional tertiary amine or a polyfunctional tertiary amine.
The present invention is also directed to a composite reverse osmosis membrane comprising a porous support and a polyamide membrane of the type broadly described above.
It is another object of the present invention to provide a method of producing a composite reverse osmosis membrane.
Consequently, according to a further aspect of the invention, there is provided a method of making a composite reverse osmosis membrane, said method comprising the steps of:
(a) coating a porous support with an aqueous solution comprising a polyfunctional amine, one or more polar solvents and a salt-containing compound, said salt-containing compound being a reaction product of a strong acid and a polyfunctional tertiary amine, so as to form a liquid layer on said porous support;
(b) contacting said liquid layer with an organic solvent solution comprising an amine-reactive reactant selected from the group consisting of a polyfunctional acyl halide, a polyfunctional sulfonyl halide and a polyfunctional isocyanate so as to interfacially condense said amine-reactive reactant with said polyfunctional amine, thereby forming a cross-linked, interfacial polyamide layer on said porous support; and
(c) drying the product of step (b) to form a composite reverse osmosis membrane.
According to yet a further aspect of the invention, there is provided a method of making a composite reverse osmosis membrane, said method comprising the steps of:
(a) coating a porous support with a first aqueous solution, said first aqueous solution comprising one or more polar solvents and an amine salt, so as to form a first liquid layer on said porous support;
(b) coating the first liquid layer with a second aqueous solution, said second aqueous solution comprising a polyfunctional amine, so as to form a second liquid layer over said first liquid layer;
(c) contacting the twice-coated porous support with an organic solvent solution comprising an amine-reactive reactant selected from the group consisting of a polyfunctional acyl halide, a polyfunctional sulfonyl halide and a polyfunctional isocyanate so as to interfacially condense said amine-reactive reactant with said polyfunctional amine, thereby forming a cross-linked, interfacial polyamide layer on said porous support; and
(d) drying the product of step (c) to form a composite reverse osmosis membrane.
According to still yet another aspect of the invention, there is provided a method of making a composite reverse osmosis membrane, said method comprising the steps of:
(a) coating a porous support with a first aqueous solution, said first aqueous solution comprising one or more polar solvents, so as to form a first liquid layer on said porous support;
(b) coating the first liquid layer with a second aqueous solution, said second aqueous solution comprising a polyfunctional amine and an amine salt, so as to form a second liquid layer over said first liquid layer;
(c) contacting the twice-coated porous support with an organic solvent solution comprising an amine-reactive reactant selected from the group consisting of a polyfunctional acyl halide, a polyfunctional sulfonyl halide and a polyfunctional isocyanate so as to interfacially condense said amine-reactive reactant with said polyfunctional amine, thereby forming a cross-linked, interfacial polyamide layer on said porous support; and
(d) drying the product of step (c) to form a composite reverse osmosis membrane.
Additional objects, features, aspects and advantages of the present invention will be set forth, in part, in the description which follows and, in part, will be obvious from the description or may be learned by practice of the invention. Certain embodiments of the invention will be described hereafter in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural or other changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.